1. Field of the Invention
The present invention relates to equipment for the production of printed circuit board and, more particularly, to a device for controlling the pressing action of a panel press with reference to the flux point of thermosetting resinous bonding layers in connection with the assembly of multilayer printed circuit boards.
2. Description of the Prior Art
The production of multilayer printed circuit boards involves the assembly of a stack of circuit boards, in alternation with resinous bonding layers and the application to the stack of controlled heat and pressure by means of a heated panel press. The latter is preferably a multi-stage press suitable for the simultaneous production of a number of multilayer printed circuit boards.
The application of controlled heat to the resinous bonding layers results in a succession of physical and chemical changes in the resin, beginning with a plastification process which reaches a flux point at which the bonding layer has a minimal viscosity and a high wetting factor for good adhesion. With increasing temperature, the resin undergoes a polycondensation process, as a result of which it gels and rapidly increases in its viscosity. The resin now undergoes a curing phase, at the end of which it is transformed into a permanently hardened, rigid state.
Different levels of pressure must be applied to the multilayer stack during this bonding layer transformation. A moderate contact pressure, or heat transfer pressure, is applied during the major part of the heat buildup phase, and a much higher curing pressure is applied in the latter part of the curing phase and during at least an initial portion of the cooling phase. The selection of the point in time at which the full curing pressure is applied to the multilayer stack is crucial for the production quality, in terms of the permanent physical characteristics obtained in the interface between the printed circuit boards and the intermediate bonding layers.
It is possible to apply the curing pressure too soon, at a point at which the viscosity of the resin in the bonding layers is still very low, i.e. at or near the "flux point", with the result that the bonding layers will be squeezed too thin and/or undesirable deformations take place in the printed circuit boards themselves. It is also possible to apply the curing pressure too late, at a point at which the gelling of the resin has progressed so far that the bonding layer is no longer susceptible to compression and flow deformation, with the result that the bond interfaces are inadequately engaged and include cavities.
Accordingly, the switchover from the contact pressure to the curing pressure should take place with a predetermined delay from the point at which the flux point has been reached. The timing of the switchover, therefore, rests on a reliable determination of the flux point during the pressing operation.
It has already been suggested that, by measuring the relative displacements between the press table and the upper press plate, it is possible to obtain a measure of the viscosity of the resin of the bonding layers, inasmuch as these layers undergo compressive deformation and the height of the multilayer stack decreases.
This suggestion is contained in a report published in the German journal "Plastverarbeiter", Volume 30 (1979), No. 9, pp. 519-23. This report suggests the connection to the press table of a friction-wheel-driven potentiometer and the comparison of the potentiometer signal with a reference signal, to obtain a compression graph of the multilayer stack and to derive from the compression graph the point at which the switchover to the curing pressure is to take place.
This method of determining the switchover point, while being satisfactory on an experimental level, is labor-intensive under production conditions, because it requires the intervention of a skilled press operator who, based on his interpretation of the compression graph produced by the displacement measuring device, has to make the decision to switch the panel press from the contact pressure level to the curing pressure level.
The prior art experimental setup also addresses itself to a source of distortion in the displacement values of the compression graph, distortions which are the result of thermal expansion of the press parts. In order to determine the extent of these distortions, the report suggests the tracing of a corrective compression graph with a set of non-compressible, fully hardened multilayers, under identical operating conditions.